Laying out of fibers for forming a web is conventionally effected by defibrating a pulp material and admixing the pulp material into an airflow, which brings the loose fibers to a distributor head above a moving, perforated forming wire. (A forming wire is a wire screen made of wires or monofilaments which cross each other in a mesh-like fashion.) Underneath this wire is a suction chamber for down suction of the fibers against the wire, where they are deposited as a coherent fibrous pulp web with a desired web thickness. The products are typically used as liquid absorbing sheets.
It is important that the dryformed pulp web be stabilized or bonded. In practice, this is achieved in two different ways, namely, by applying binder or by using binding fibers.
Binding
The pulp web is passed through a bonding station in which a suitable glue, normally of the latex type, is sprayed onto the web through a plurality of nozzles. After that, the wire is led through a tunnel over for drying the binder. Thereafter, the web is transferred to an overhead wire, against which it is fixed by suction from above, so that the underside of the web is exposed. The web is then led through or over another bonding station, where binder is correspondingly sprayed against the underside of the web. Thereafter, the web is led through a tunnel over for drying and final setting of the binder, after which the web is self supporting and can be wound up for storage or delivery
This method provides sheet webs of rather high quality, but the method requires machinery which is exact. The method is thus vulnerable to irregularities in the binding stations. It is another limitation that the method is not suitable for forming thick sheet webs, as it is difficult to achieve a deep penetration of the binder, so that a relatively thick layer tends to segregate in the center plane, where bonding is weak or is completely lacking. The products typically contain about 85% cellulose fibers and about 15% binding agent. Typical weights of these products is on the order to 50–120 g/m2, as heavier products easily delaminate. Furthermore, the products are not suited for containing fibers and superabsorbent fibers.
It is well known that when binder fluid is sprayed onto an airlaid web of cellulose fibers derived from wood, the binder penetration into the web can vary within certain limits. It should be noted that the binder must be sprayed onto the web, because the web prior to having binder added does not have any cohesion and does not tolerate direct contact from the application equipment. Because the cellulose fibers have a large specific surface area and a high capacity for liquid absorption, the web works as a kind of filter, and the particles of the binder tend to become absorbed in the surface of the web, particularly if the diameter of the particles is less than 0.05 mm. This means that bonded products have a tendency to delaminate at a weight of approximately 70 g/m2.
In order to avoid delamination, the web can be compacted or embossed before the binder is sprayed on. The airflow through the fiber web is also increased during binder spraying. The application by means of airless atomization equipment also increases the penetration of the binder into the web considerably. The binder in this case is atomized at a pressure of from about 10 bar to about 15 bar, and particles of a size up to about 0.5 mm are “shot” into the fiber web. Nevertheless, the delamination limit for products bonded in this way is still at a product weight of only 100 to 120 g/m2. Furthermore, this bonding method has the disadvantage that a large proportion of the cellulose fibers are coated with a binder film, which delays and reduces the ability of the product to absorb liquids.
Normally, products bonded by spraying followed by compacting or embossing are relatively compact, lint free, and have a relatively high durability and reasonably good absorption. These products are mostly used to make napkins, wet tissues, table cloths, kitchen towels, and the like.
Binding Fibers
In this method, a homogeneous admixture of heat actuated binding fibers, also known as thermobonding fibers, is made with cellulose fibers. The dryformed web material is fixed solely by being led through a heating zone. As to system and control, this method is more simple than the above binding method. Thick webs can be manufactured by this process, as the bonding fibers are evenly present in the outer as well as in the inner planes of the material. The weight of product may typically be 40–800 g/m2, and these webs can incorporate a considerable amount of superabsorbent agent or other additives. The quality achieved by this method is fully acceptable for many different applications. The typical content of thermobonding fibers in the product is 15%. A higher content of thermobonding fibers would reduce the absorption capabilities of the product, but would also be economically unattractive.
Normally, the cellulose fibers range in lengths between 0.5 and approximately 5 mm, whereas heat thermobonding fibers are often 6 mm or longer. In practice, this results in very good bonding of the longest cellulose fibers. This means that a certain amount of dust develops in the subsequent converting process, during which the customer converts the airlaid material into end products such as hygiene and incontinence products. During this process the airlaid material is wrapped with a coverstock and other material, which prevents the insufficiently bonded cellulose fibers from leaving the product. The converting is carried out at a line speed of up to about 250 m/minute, and the thermobonded airlaid product will here be exposed to both vibration and reverse stretching. The dust which breaks loose during their influence may cause serious hygienic problems for the workers. The application of a strong exhaust system, connected to a filtration system, can solve most of these problems, but this is an expensive installation, as the air which is sucked out of the production area must be replaced by conditioned air. This system is also expensive to run. Another and a better solution to the dust problem is treatment of the airlaid material during manufacture in order to reduce the development of dust to such an extent that dust problems do not occur during converting.
Airlaid products bonded with thermal bonding fibers have different properties from products made solely with binding agent sprayed onto the web. For products with thermal bonding fibers, there is no delamination limit because the fibers are homogeneously distributed throughout the product. For instance, it is possible to manufacture a product of 1000 g/m2. It is not necessary to compact the web very much, and the cellulose fibers are not coated with binders.
Because of their good absorption properties, thermally bonded airlaid products made from cellulose derived from wood, often together with other additives such as superabsorbent, are mainly used as functional shields inside hygienic products such as sanitary napkins, incontinence products, etc.
Airlaid products made from cellulose derived from wood bonded with thermally activated bonding fibers, however, have a disadvantage compared to binder bonded products. Because wood cellulose has a large content of very short fibers, and also because, for economic and quality reasons, no more than 15 to 25% of thermally activated bonding fibers are used, the cellulose fibers are not bonded 100%. Therefore, the products have a tendency to release fibers (i.e., lint) when exposed to stretching and vibration during further handling.
It has been thought to avoid dusting by using a higher percentage of binder fibers of different lengths, this solution is hardly realistic from a production standpoint.